A light-emitting element using an organic compound as a luminous body, which has features such as thinness, lightness, high-speed response, and DC drive at low voltage, is expected to be applied to a next-generation flat panel display. In particular, a display device in which light-emitting elements are arranged in matrix is considered to have advantages in a wide viewing angle and excellent visibility over a conventional liquid crystal display device.
It is said that, as for a light-emitting mechanism of a light-emitting element, an EL layer is interposed between a pair of electrodes and voltage is applied to the EL layer, so that electrons injected from a cathode and holes injected from an anode are recombined in an emission center of the EL layer to form molecular excitons, and the molecular excitons release energy when returning to a ground state; thus, light is emitted. Singlet excitation and triplet excitation are known as excitation states, and it is considered that light can be emitted through either of the two excitation states.
An EL layer included in a light-emitting element includes at least a light-emitting layer. In addition, the EL layer can have a stacked-layer structure including a hole-injecting layer, a hole-transporting layer, an electron-transporting layer, an electron-injecting layer, and/or the like, in addition to the light-emitting layer.
EL materials for forming an EL layer are broadly classified into a low molecular (monomer) material and a high molecular (polymer) material. In general, a low molecular material is often deposited by an evaporation method and a high molecular material is often deposited by an inkjet method, a spin coating method, or the like.
An evaporation apparatus which is used for an evaporation method has a substrate holder to which a substrate is mounted; a crucible (or an evaporation boat) containing an EL material, that is, an evaporation material; a heater for heating the EL material in the crucible; and a shutter for preventing the EL material from being scattered during sublimation. The EL material which is heated by the heater is sublimed and deposited on the substrate. In order to achieve uniform deposition, a deposition target substrate needs to be rotated and the distance between the substrate and the crucible needs to be about 1 m even when the substrate has a size of 300 mm×360 mm. Accordingly, when a substrate to be processed is larger, an evaporation apparatus also needs to be larger; therefore, it can be thought that there is practically a limitation on the size of a substrate on which deposition can be performed by an evaporation method.
When an evaporation method is employed to manufacture a full-color display device using light emitting elements of red, green, and blue, a shadow mask is provided between the substrate and an evaporation source so as to be in contact with the substrate, and selective coloring can be achieved with this shadow mask.
However, the shadow mask which is used for manufacturing the full-color display device is extremely thin since it is necessary to precisely manufacture an opening. Therefore, when the shadow mask size is increased in accordance with an increase in a substrate size, there have been problems of bending of the shadow mask, changing of the size of the opening, and the like. Furthermore, since it is difficult to introduce a means for reinforcing the strength of the shadow mask in a region which corresponds to a pixel portion of the shadow mask, in the case of manufacturing a display region having a large area, application of a reinforcing means is also difficult.
Further, miniaturization of each display pixel pitch is increasingly demanded with high definition of a display device (increase in the number of pixels), and the shadow mask tends to be thin.
On the other hand, a wet process such as an ink-jet method or a spin coating method can be used even when a substrate is larger; however, it is difficult to form an even film. Further, when a wet process is used, after application of a composition or a solution containing an EL material, a baked solvent needs to be removed. Accordingly, when layers containing an EL material are stacked, repetition of an application step and a baking step is needed, which takes much time. Further, when a layer is stacked by a wet process such as an ink-jet method, deposition has to be performed using a solvent which does not dissolve a layer which has already been formed, and the selection range of materials to be used or stacked-layer structures is limited. When the selection range of materials to be used or stacked-layer structures is limited, performance of a light-emitting element (luminous efficiency, lifetime, or the like) is extremely limited. Accordingly, a large defect is caused in improvement in performance of a light-emitting device, such as being unable to apply a light-emitting element having an excellent structure to a light-emitting device.
Thus, a method for forming an EL layer of a light-emitting element through laser thermal transfer has been proposed (see Patent Document 1: Japanese Published Patent Application No. 2006-309995). Patent Document 1 discloses a transfer substrate which has a photothermal conversion layer including a low-reflective layer and a high-reflective layer and a transfer layer over a supporting substrate. Irradiation of such a transfer substrate with laser light allows the transfer layer to be transferred to an element-forming substrate.